Substituted indolocarbazoles

ABSTRACT

A substituted indolocarbazole comprising at least one optionally substituted thienyl.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/366,857, filed Feb. 6, 2012, now U.S. patent Ser. No. [insert later],which is a continuation of U.S. patent application Ser. No. 12/252,766,filed Oct. 16, 2008, now U.S. Pat. No. 8,110,690, which was a divisionalapplication of co-pending U.S. application Ser. No. 11/167,485, filedJun. 27, 2005, now U.S. Pat. No. 7,456,424, which was acontinuation-in-part application of U.S. patent application Ser. No.11/011,441, filed Dec. 14, 2004, the disclosures of which are totallyincorporated herein by reference.

Beng S. Ong et al., U.S. application Ser. No. 11/167,512 (AttorneyDocket No. A3571-US-CIP), filed on Jun. 27, 2005, titled “COMPOUND WITHINDOLOCARBAZOLE MOIETIES AND DEVICES CONTAINING SUCH COMPOUND” is acontinuation-in-part application of Beng S. Ong et al., U.S. applicationSer. No. 11/011,678 (Attorney Docket No. A3571-US-NP), filed on Dec. 14,2004, from which priority is claimed, the disclosure of which is totallyincorporated herein by reference.

Yuning Li et al., U.S. application Ser. No. 11/011,901 (Attorney DocketNo. 20031573-US-NP), filed on Dec. 14, 2004, titled “PROCESS TO FORMCOMPOUND WITH INDOLOCARBAZOLE MOIETIES.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underCooperative Agreement No. 70NANBOH3033 awarded by the National Instituteof Standards and Technology (NIST). The United States Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

Organic thin-film transistors (OTFTs) have attracted much attention inrecent years as a low-cost alternative to amorphous silicon transistorsfor electronic applications. OTFTs are particularly suited forapplications where large-area circuits (e.g., backplane electronics forlarge displays), desirable form factors and structural features (e.g.,flexibility for e-paper), and affordability (e.g., ultra low cost forubiquitous RFID tags) are essential. The key component of OTFTs is thesemiconductor materials. Therefore, there is enormous interest in thedevelopment of new organic semiconductors for OTFT applications.

Organic semiconductors are typically based on: (1) acenes such astetracene, pentacene and their derivatives, (2) thiophenes such asoligothiophenes and polythiophenes, (3) fused-ring thiophene-aromaticsand thiophene-vinylene/arylene derivatives. Most of these semiconductorsare either insoluble or are sensitive to air, and may therefore not besuitable for low-cost OTFT applications. Therefore, there is a needaddressed by embodiments of the present invention to develop new organicsemiconductor compounds that can be processed in air for manufacturinglow-cost OTFTs.

Moreover, it is desirable that the charge carrier mobility of OTFTsapproach that of the amorphous silicon transistor. There is therefore aneed addressed by embodiments of the present invention to providesufficiently high charge carrier mobility for OTFTs.

The following documents provide background information:

-   -   Christos D. Dimitrakopoulos et al., “Organic Thin Film        Transistors for Large Area Electronics,” Adv. Mater., Vol. 14,        No. 2, pp. 99-117 (2002).    -   Salem Wakim et al., “Organic Microelectronics: Design,        Synthesis, and Characterization of        6,12-Dimethylindolo[3,2-b]Carbazoles,” Chem. Mater. Vol. 16, No.        23, pp. 4386-4388 (published on web Jul. 7, 2004).    -   Nan-Xing Hu et al.,        “5-11-Dihydro-5,11-di-1-naphthylindolo[3,2-b]carbazole:        Atropisomerism in a Novel Hole-Transport Molecule for Organic        Light-Emitting Diodes,” J. Am. Chem. Soc., Vol. 121, pp.        5097-5098 (1999).    -   Hu et al., U.S. Pat. No. 5,942,340.    -   Hu et al., U.S. Pat. No. 5,952,115.    -   Hu et al., U.S. Pat. No. 5,843,607.

SUMMARY OF THE DISCLOSURE

In embodiments, there is provided a substituted indolocarbazolecomprising at least one optionally substituted thienyl.

In additional embodiments, there is provided an electronic devicecomprising a substituted indolocarbazole comprising at least oneoptionally substituted thienyl.

In embodiments, there is provided a thin film transistor comprising:

(a) a gate dielectric layer;

(b) a gate electrode;

(c) a semiconductor layer including an optionally substitutedindolocarbazole;

(d) a source electrode; and

(e) a drain electrode,

wherein the gate dielectric layer, the gate electrode, the semiconductorlayer, the source electrode, and the drain electrode are in any sequenceas long as the gate electrode and the semiconductor layer both contactthe gate dielectric layer, and the source electrode and the drainelectrode both contact the semiconductor layer.

In further embodiments, there is provided a thin film transistorcomprising:

(a) a gate dielectric layer;

(b) a gate electrode;

(c) a semiconductor layer;

(d) a source electrode; and

(e) a drain electrode,

wherein the gate dielectric layer, the gate electrode, the semiconductorlayer, the source electrode, and the drain electrode are in any sequenceas long as the gate electrode and the semiconductor layer both contactthe gate dielectric layer, and the source electrode and the drainelectrode both contact the semiconductor layer, andwherein the semiconductor layer includes an indolocarbazole selectedfrom the group consisting of structures (A), (B), (C), (D), (E), (F),and (G), or a mixture thereof:

wherein for each of the structures (A) through (G), each R isindependently selected from a group consisting of a hydrogen, ahydrocarbon group and a heteroatom-containing group, wherein each of thestructures (A) through (G) is optionally peripherally substituted.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the present invention will become apparent as thefollowing description proceeds and upon reference to the followingfigures which represent exemplary embodiments:

FIG. 1 represents a first embodiment of the present invention in theform of an OTFT;

FIG. 2 represents a second embodiment of the present invention in theform of an OTFT;

FIG. 3 represents a third embodiment of the present invention in theform of an OTFT; and

FIG. 4 represents a fourth embodiment of the present invention in theform of OTFT.

Unless otherwise noted, the same reference numeral in different Figuresrefers to the same or similar feature.

DETAILED DESCRIPTION

The term “indolocarbazole” refers to a structure composed of onecarbazole moiety (optionally substituted) and one, two or more indolomoieties (each optionally substituted), wherein the carbazole moiety isfused with one or more of the indolo moieties, and any adjacent indolomoieties are fused together. The fusing of the carbazole moiety with theone or more indolo moieties, and the fusing of adjacent indolo moietiescan occur at any available positions. The carbazole moiety may bepositioned at any suitable position in the structure such as at the endor the middle of the structure.

The optionally substituted indolocarbazole (a single compound or amixture of two or more indolocarbazoles, each independently optionallysubstituted) may be used for any suitable applications, particularly asa semiconductor for electronic devices. The phrase “electronic devices”refers to macro-, micro- and/or nano-electronic devices such as thinfilm transistors, organic light emitting diodes, RFID tags,photovoltaic, and other electronic devices. In embodiments, theelectronic device comprises a substituted indolocarbazole comprising atleast one optionally substituted thienyl.

In embodiments, the indolocarbazole is unsubstituted or substituted withone or more substituents in any suitable substitution pattern. Forsubstituted embodiments of the indolocarbazole, the substitution can befor example the following: (1) one or more nitrogen substitutions withno peripheral substitution; (2) one or more peripheral substitutionswith no nitrogen substitution; or (3) one or more nitrogen substitutionsand one or more peripheral substitutions. In embodiments, all thenitrogen atoms of the indolocarbazole are substituted with the same ordifferent substituents, with the indolocarbazole being optionallyperipherally substituted. In embodiments, the indolocarbazole isnitrogen substituted (and optionally peripherally substituted) whereinthe one or more nitrogen substituents are independently selected fromthe group consisting of a hydrocarbon group and a heteroatom-containinggroup, or a mixture thereof. In embodiments, the indolocarbazole isperipherally substituted (and optionally nitrogen substituted) whereinthe one or more peripheral substituents are independently selected fromthe group consisting of a hydrocarbon group, a heteroatom-containinggroup, and a halogen, or a mixture thereof.

In embodiments, the indolocarbazole is independently selected from thegroup consisting of structures (A), (B), (C), (D), (E), (F), and (G), ora mixture thereof:

wherein for each of the structures (A) through (G), each R isindependently selected from a group consisting of a hydrogen, ahydrocarbon group and a heteroatom-containing group (that is, eachnitrogen atom can have the same or different R), wherein each of thestructures (A) through (G) is optionally peripherally substituted by oneor more substituents selected from the group consisting of a hydrocarbongroup, a heteroatom-containing group, and a halogen, or a mixturethereof.

The phrases “peripherally substituted” and “peripheral substitution”refer to at least one substitution (by the same or differentsubstituents) on any one or more aromatic rings of the indolocarbazoleregardless whether the aromatic ring is a terminal aromatic ring or aninternal aromatic ring (that is, other than at a terminal position).

The hydrocarbon group for the optionally substituted indolocarbazolecontains for example from 1 to about 50 carbon atoms, or from 1 to about30 carbon atoms, and may be for example a straight chain alkyl group, abranched alkyl group, a cyclic aliphatic group, an aryl group, analkylaryl group, and an arylalkyl group. Exemplary hydrocarbon groupsinclude for example methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl,octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,cyclopentyl, cyclohexyl, cycloheptyl, and isomers thereof.

The heteroatom-containing group for the optionally substitutedindolocarbazole has for example 2 to about 200 atoms, or from 2 to about100 atoms and may be for example a nitrogen containing group, an alkoxygroup, a heterocyclic system, an alkoxyaryl, an arylalkoxy, and ahalogenated hydrocarbon (where the halogen is for example fluorine,bromine, chlorine, or iodine, or a mixture thereof). Exemplaryheteroatom-containing groups include for example fluoroalkyl,fluoroaryl, cyano, nitro, carbonyl, carboxylate, amino (optionallysubstituted with one or two substituents such as for example ahydrocarbon group described herein), and alkoxy (having for example 1 toabout 18 carbon atoms). In embodiments, the heteroatom-containing groupis independently selected from fluoroalkyl (having for example 1 toabout 18 carbon atoms), fluoroaryl, alkoxy (having for example 1 toabout 18 carbon atoms), carbonyl, carboxylate, amino (optionallysubstituted with one or two substituents such as for example ahydrocarbon group described herein), nitro, and cyano, or a mixturethereof. In embodiments, the heteroatom-containing group is anoptionally substituted carbazole group.

In embodiments, the heteroatom-containing group is an optionallysubstituted thienyl comprising one, two, or more thienyl units, eachthienyl unit being the same or different from each other and ofexemplary structure (A)

where R₁ is independently selected from a hydrocarbon group (such asthose described herein for the optionally substituted indolocarbazole),a heteroatom containing group (such as those described herein for theoptionally substituted indolocarbazole), and a halogen (such as thosedescribed herein for the optionally substituted indolocarbazole), andwhere m is 0, 1, 2 or 3.

In embodiments, R₁ is part of a cyclic ring structure fused to thethienyl unit, where the fused cyclic ring structure is of any size suchas for example a 4 to 8 membered ring, particularly, a 5 or 6 memberedring, wherein R₁ is attached at the carbon atoms at for example thethird ring position and the fourth ring position of the thienyl unit, orany other available ring positions. The fused ring structure (containingR₁) may be either a hydrocarbon group described herein or a heteroatomcontaining group described herein. Where R₁ is part of a ring structurefused to a thienyl unit, m is 1 even though R₁ in this situation isbonded to two positions of the thienyl unit. Examples of the thienylunit with R₁ being part of a ring substituent structure are thefollowing:

wherein R₂ and R₃ are the same or different from each other, and areselected from the group consisting of:

-   -   (a) a hydrocarbon group (such as those described herein for the        optionally substituted indolocarbazole),    -   (b) a heteroatom containing group (such as those described        herein for the optionally substituted indolocarbazole),    -   (c) a halogen (such as those described herein for the optionally        substituted indolocarbazole), and    -   (d) a hydrogen.

The halogen for the optionally substituted indolocarbazole is fluorine,bromine, chlorine, or iodine, or a mixture thereof.

The number of the indolo moiety in the optionally substitutedindolocarbazole is for example from 1 to 4, or from 1 to 2.

To be an efficient semiconductor for OTFTs, the optionally substitutedindolocarbazole in embodiments provides (i) proper molecular orderingconducive to charge carrier transport; and (ii) sufficient stabilizationto charge carriers to enable efficient charge carrier injection. Inembodiments, the indolocarbazole has one or more strategically placedsubstituents comprising for example at least one long chain alkyl group(having for example about 6 to about 18 carbon atoms in length) topromote molecular self-assembly, thus forming proper molecular orderingfor charge carrier transport. In embodiments, the indolocarbazole alsohas one or more strategically placed substituents such as for examplearyl substituents at the nitrogen positions to provideresonance-stabilization to injected charge carriers. In embodiments, toprovide resonance-stabilization to injected charge carriers, theindolocarbazole is substituted with one or more substituentsindependently selected from the group consisting of a long chain alkylgroup (having for example about 6 to about 18 carbon atoms in length),an alkylphenyl (the alkyl of the alkylphenyl having for example about 6to about 18 carbon atoms in length), a phenyl, a chloro, an alkoxy(having for example 1 to about 18 carbon atoms), and an amino(optionally substituted with one or two substituents such as for examplea hydrocarbon group described herein), or a mixture thereof.

The indolocarbazole may be a p-type semiconductor or n-typesemiconductor, depending very much on the nature of the substituents.Substituents which possess an electron donating property such as alkyl,alkoxy and aryl groups, when present in the indolocarbazole, render theindolocarbazole a p-type semiconductor. On the other hand, substituentswhich are electron withdrawing such as cyano, nitro, fluorinated alkyl,and fluorinated aryl groups may transform the indolocarbazole into then-type semiconductor.

In embodiments, the optionally substituted indolocarbazole has a bandgap of for example greater than about 1.8 eV, greater than about 2.0 eV,or greater than about 2.5 eV. The corresponding highest occupiedmolecular orbital (HOMO) energy level of the optionally substitutedindolocarbazole is for example lower than about 4.9 eV from vacuum,preferably lower than about 5.1 eV from vacuum. The optionallysubstituted indolocarbazoles are in embodiments relatively stableagainst oxygen doping in air by virtue of their relatively low lyingHOMOs.

In embodiments, the relatively low-lying HOMOs and large band gaps ofthe optionally substituted indolocarbazole generally provides manyadvantages over other semiconductors. For example, in embodiments, theoptionally substituted indolocarbazoles generally have no or littleabsorbance in the visible region of the spectrum, and are thereforeexpected to be photochemically stable when exposed to light.

Illustrative examples of the optionally substituted indolocarbazolewhich may be suitable for OTFT applications are shown in the following:

or a mixture thereof.

Any suitable methods can be used to prepare the optionally substitutedindolocarbazole. For example, indolocarbazoles of compound (1) andcompound (2) can be prepared as follows. The5,11-dihydridoindolo[3,2-b]carbazole could be first prepared by doubleFischer indolization starting from phenylhydrazine and1,4-cyclohexanedione according to the method described in B. Robinson,J. Chem. Soc. 1963, pp. 3097-3099, the disclosure of which is totallyincorporated herein by reference. The5,11-dialkylindolo[3,2-b]carbazole, such as compound (1), was readilyprepared by phase-transfer condensation of5,11-dihydridoindolo[3,2-b]carbazole with alkylbromide and aqueous NaOHin dimethyl sulfoxide (“DMSO”) in the presence of a phase transfercatalyst, benzyltriethylammonium chloride. On the other hand,5,11-diarylindolo[3,2-b]carbazole, such as compound (2), was obtained byUllmann condensation of 5,11-dihydridoindolo[3,2-b]carbazole witharyliodide using excess copper and a catalytic amount of 18-crown-6 inrefluxing 1,2-dichlorobenzene.

Any suitable fabrication techniques may be used to form thesemiconductor layer including the optionally substituted indolocarbazolefor OTFTs. One such method is by vacuum evaporation at a vacuum pressureof about 10⁻⁵ to 10⁻⁷ torr in a chamber containing a substrate and asource vessel that holds the optionally substituted indolocarbazoles.The vessel is heated until the compound sublimes and deposits on thesubstrate. In embodiment, the substrate may be held at room temperatureor at an elevated temperature of for example 50° C., 70° C., or 120° C.Solution deposition techniques, in embodiments, may also be used tofabricate the semiconductor layer including the optionally substitutedindolocarbazole. Solution deposition techniques refer to spin coating,blade coating, rod coating, screen printing, ink jet printing, stampingand the like. As an example of liquid deposition, the optionallysubstituted indolocarbazole is first dissolved in a suitable solvent offor example tetrahydrofuran, dichloromethane, chlorobenzene,dichlorobenzene, toluene, and xylene to form a solution with aconcentration of about 0.1% to 30%, particularly about 0.3% to 10% byweight. The solution is then used to form a semiconductor layer on asuitable substrate via spin coating at a speed of about 500 to about3000 rpm, particularly about 1000 to about 2000 rpm for a period of timeof about 5 to about 180 seconds, particularly about 30 to about 60seconds at room temperature or an elevated temperature.

The semiconductor layer may be predominantly amorphous or predominantlycrystalline in nature, depending on the indolocarbazole and processingconditions. The semiconductor layer can be characterized by commoncharacterization techniques such as X-ray diffraction, atomic forcemicroscopy, optical microscopy, etc. For example, a predominantlyamorphous layer usually shows broad X-ray diffraction peaks, while apredominantly crystalline layer generally exhibits sharp X-raydiffraction peaks. The degree of crystallinity of a semiconductor layercan be calculated from the integrated area of diffraction peaks. Inembodiments, the phrase “predominately crystalline” indicates that thecrystallinity of the semiconductor layer is for example larger thanabout 50%, larger than about 80%, or larger than about 90%.

Depending on the nature of the optionally substituted indolocarbazole, apredominantly crystalline semiconductor layer can be formed by a numberof techniques. For example, a predominantly crystalline semiconductorlayer can be formed by vacuum evaporation of the indolocarbazole onto asubstrate holding at an elevated temperature of for example about 50° C.to about 120° C. In another technique, a predominantly crystallinesemiconductor layer can be achieved by solution coating followed bycontrolled solvent evaporation and optionally post-deposition annealingat an elevated temperature of for example about 80° C. to about 250° C.

In FIG. 1, there is schematically illustrated an OTFT configuration 10comprised of a substrate 16, in contact therewith a metal contact 18(gate electrode) and a gate dielectric layer 14 on top of which twometal contacts, source electrode 20 and drain electrode 22, aredeposited. Over and between the metal contacts 20 and 22 is an organicsemiconductor layer 12 as illustrated herein.

FIG. 2 schematically illustrates another OTFT configuration 30 comprisedof a substrate 36, a gate electrode 38, a source electrode 40 and adrain electrode 42, a gate dielectric layer 34, and an organicsemiconductor layer 32.

FIG. 3 schematically illustrates a further OTFT configuration 50comprised of a heavily n-doped silicon wafer 56 which acts as both asubstrate and a gate electrode, a thermally grown silicon oxide gatedielectric layer 54, and an organic semiconductor layer 52, on top ofwhich are deposited a source electrode 60 and a drain electrode 62.

FIG. 4 schematically illustrates an additional OTFT configuration 70comprised of substrate 76, a gate electrode 78, a source electrode 80, adrain electrode 82, an organic semiconductor layer 72, and a gatedielectric layer 74.

The composition and formation of the semiconductor layer are describedherein.

The semiconductor layer has a thickness ranging for example from about10 nanometers to about 5 micrometer with a preferred thickness of fromabout 20 to about 500 nanometers. The OTFT devices contain asemiconductor channel with a width W and length L. The semiconductorchannel width may be, for example, from about 1 micrometer to about 5millimeters, with a specific channel width being about 5 micrometers toabout 1 millimeter. The semiconductor channel length may be, forexample, from about 500 nanometers to about 1 millimeter with a morespecific channel length being from about 1 micrometer to about 100micrometers.

The substrate may be for instance a silicon wafer, glass plate, plasticfilm or sheet. For structurally flexible devices, a plastic substratesuch as for example polyester, polycarbonate, or polyimide sheet and thelike may be preferred. The thickness of the substrate may be from about10 micrometers to over about 10 millimeters with an exemplary thicknessbeing from about 50 to about 100 micrometers, especially for a flexibleplastic substrate and from about 1 to about 10 millimeters for a rigidsubstrate such as glass or silicon.

The gate electrode can be a thin metal film, a conducting polymer film,a conducting film made from conducting ink or paste, or the substrateitself can be the gate electrode, for example heavily doped siliconwafer. Examples of gate electrode materials include but are notrestricted to aluminum, gold, chromium, indium tin oxide, conductingpolymers such as polystyrene sulfonate-dopedpoly(3,4-ethylenedioxythiophene) (PSS-PEDOT), conducting ink/pastecomprised of carbon black/graphite or colloidal silver dispersion inpolymer binders, such as ELECTRODAGT™ available from Acheson ColloidsCompany. The gate electrode layer can be prepared by vacuum evaporation,sputtering of metals or conductive metal oxides, coating from conductingpolymer solutions or conducting inks by spin coating, casting orprinting. The thickness of the gate electrode layer ranges for examplefrom about 10 to about 200 nanometers for metal films and in the rangeof about 1 to about 10 micrometers for polymer conductors. The sourceand drain electrode layers can be fabricated from materials whichprovide a low resistance ohmic contact to the semiconductor layer.Typical materials suitable for use as source and drain electrodesinclude those of the gate electrode materials such as gold, silver,nickel, aluminum, platinum, conducting polymers and conducting inks.Typical thicknesses of source and drain electrodes are about, forexample, from about 40 nanometers to about 10 micrometers with the morespecific thickness being about 100 to about 400 nanometers.

The gate dielectric layer can be composed of inorganic materials,organic materials, or organic-inorganic hybrid materials. Illustrativeexamples of inorganic materials suitable as the gate dielectric layerinclude silicon oxide, silicon nitride, aluminum oxide, barium titanate,barium zirconium titanate and the like; illustrative examples of organicmaterials for the gate dielectric layer include polyesters,polycarbonates, poly(vinyl phenol), polyimides, polystyrene,poly(methacrylate)s, poly(acrylate)s, epoxy resin and the like. Thethickness of the gate dielectric layer is, for example from about 10nanometers to about 500 nanometers depending on the dielectric constantof the dielectric material used. An exemplary thickness of the gatedielectric layer is from about 100 nanometers to about 500 nanometers.The gate dielectric layer may have a conductivity of for example lessthan about 10⁻¹² S/cm.

In embodiments, the gate dielectric layer layer, the gate electrode, thesemiconductor layer, the source electrode, and the drain electrode areformed in any sequence with the gate electrode and the semiconductorlayer both contact the gate dielectric layer, and the source electrodeand the drain electrode both contact the semiconductor layer. The phrase“in any sequence” includes sequential and simultaneous formation. Forexample, the source electrode and the drain electrode can be formedsimultaneously or sequentially. The composition, fabrication, andoperation of OTFTs are described in Bao et al., U.S. Pat. No. 6,107,117,the disclosure of which is totally incorporated herein by reference.

The source electrode is grounded and a bias voltage of generally, forexample for p-channel OTFTs, about 0 volt to about −80 volts is appliedto the drain electrode to collect the charge carriers transported acrossthe semiconductor channel when a voltage of generally about +20 volts toabout −80 volts is applied to the gate electrode.

In embodiments, the semiconductor layer of the present invention whenused in OTFTs has a field-effect mobility of greater than for exampleabout 10⁻³ cm²/Vs, preferably greater than for example about 10⁻²cm²/Vs. OTFTs produced by the present process have an on/off ratio ofgreater than for example about 10³. On/off ratio refers to the ratio ofthe source-drain current when the transistor is on to the source-draincurrent when the transistor is off.

The invention will now be described in detail with respect to specificexemplary embodiments thereof, it being understood that these examplesare intended to be illustrative only and the invention is not intendedto be limited to the materials, conditions, or process parametersrecited herein. All percentages and parts are by weight unless otherwiseindicated. As used herein, room temperature refers to a temperatureranging for example from about 20 to about 25° C.

Example 1 (a) Synthesis of 5,11-Dioctylindolo[3,2-b]carbazole (1)

5,11-Dihydridoindolo[3,2-b]carbazole was synthesized by double Fischerindolization starting from phenylhydrazine and 1,4-cyclohexanedioneaccording to the method described in B. Robinson, J. Chem. Soc. 1963,3097-3099.

A freshly prepared 50% aqueous NaOH solution (4 mL) was added to awell-stirred mixture of 5,11-dihydridoindolo[3,2-b]carbazole, (0.513 g,2 mmol), benzyltriethylammonium chloride (0.09 g, 0.4 mmol),1-bromooctane (1.55 g, 8 mmol), and DMSO (20 mL) in a 100-mL flask underan argon atmosphere. The mixture was stirred at room temperature for 2.5h and then heated to 65° C. and maintained at this temperature for 4 h.Subsequently the reaction mixture was cooled down to room temperatureand poured into 200 mL methanol with stirring. The precipitated yellowsolid was filtered off and washed with water, and 3 times each withN,N-dimethylformamide, methanol, and acetone. The yellow solid waspurified by column chromatography on silica gel using hexane as eluent.Recrystallization from hexane yielded 0.867 g (90.1%) of5,11-dioctylindolo[3,2-b]carbazole, (1), which was subject to trainsublimation to obtain electrically pure material for OTFT fabrication.DSC showed two endotherms at 84° C. and 101° C. on heating. ¹H NMR(CDCl₃): δ 8.21 (d, J=7.6 Hz, 2H), 8.01 (s, 2H), 7.47 (dd, J₁=7.0 Hz,J₂=0.9 Hz, 2H), 7.41 (d, J=7.6 Hz, 2H), 7.21 (dd, J₁=7.0 Hz, J₂=0.9 Hz,2H), 4.40 (t, J=7.3 Hz, 4H), 1.95 (pent, J=7.3 Hz, 4H), 1.25-1.50 (m,20H), 0.86 (m, 6H); IR (NaCl): 3050, 2952, 2921, 2852, 1510, 1469, 1482,1326, 737 cm⁻¹.

(b) OTFT Device Fabrication and Evaluation

A top-contact thin film transistor configuration as schematicallyillustrated, for example, in FIG. 3 was selected as our test devicestructure. The test device was built on a n-doped silicon wafer with athermally grown silicon oxide layer with a thickness of about 110nanometers thereon, and had a capacitance of about 30 nF/cm² (nanofaradsper square centimeter), as measured with a capacitor meter. The waferfunctioned as the gate electrode while the silicon oxide layer acted asthe gate dielectric. The silicon wafer was first cleaned withisopropanol, argon plasma, isopropanol and air dried, and then immersedin a 0.1 M solution of octyltrichlorosilane (OTS-8) in toluene at 60° C.for 20 min. Subsequently, the wafer was washed with toluene, isopropanoland air-dried. A 100-nm thick semiconductor layer of indolocarbazole (1)was deposited on the OTS-8-treated silicon wafer substrate by vacuumevaporation at a rate of 2 Å/s under a high vacuum of 10⁻⁶ torr with thesubstrate held at room temperature. Thereafter, the gold source anddrain electrodes of about 50 nanometers were deposited on top of thesemiconductor layer by vacuum deposition through a shadow mask withvarious channel lengths and widths, thus creating a series oftransistors of various dimensions.

The predominately crystalline nature of vacuum evaporated thin film ofindolocarbazole (1) was confirmed by X-ray diffraction measurement.Sharp diffraction peak in their X-ray diffraction patterns was observedat 2θ=5.24° with the second and third diffraction peaks at 2θ=10.48° and15.80°, corresponding to an interlayer distance of 16.85 Å.

The performance of the OTFT devices with semiconductor indolocarbazole(1) was characterized using a Keithley 4200 SCS semiconductorcharacterization system in a black box (that is, a closed box whichexcluded ambient light) at ambient conditions. The field-effectmobility, μ, was calculated from the data in the saturated regime (gatevoltage, V_(G)<source-drain voltage, V_(SD)) according to equation (1)=

I _(SD) =C _(i)μ(W/2L)(V _(G) −V _(T))²  (1)

where I_(SD) is the drain current at the saturated regime, W and L are,respectively, the semiconductor channel width and length, Ci is thecapacitance per unit area of the gate dielectric layer, and V_(G) andV_(T) are, respectively, the gate voltage and threshold voltage. V_(T)of the device was determined from the relationship between the squareroot of I_(SD) at the saturated regime and V_(G) of the device byextrapolating the measured data to I_(SD)=0.

The transfer and output characteristics of the devices showed thatindolocarbazole (1) is a p-type semiconductor. Using transistors with adimension of W=5,000 μm and L=90 μm, the following average propertiesfrom at least five transistors were obtained:

Mobility: 1.3−3.0×10⁻³ cm²/Vs

On/off ratio: 10⁴-10⁵.

Example 2 (a) 5,11-Bis(4-octylphenyl)indolo[3,2-b]carbazole (2)

1-Iodo-4-octylbenzene was first prepared as follows. A mixture of1-phenyloctane (14.87 g, 78.13 mmol), iodine (7.93 g, 31.25 mmol), H₅IO₆(3.56 g, 15.63 mmol), acetic acid (40 mL), deionized water (7 mL), and98% sulfuric acid (2.59 g) in a 100 mL flask was heated at 80° C. forabout 3 h until the purple iodine color disappeared. The reactionmixture was extracted with dichloromethane, neutralized with saturatedaqueous NaHCO₃, and washed three times with water. The organic layer wasseparated, dried over MgSO₄, filtered, and the solvent was removed usinga rotary evaporator. After column chromatography on silica gel usinghexane, 22.53 g of a colorless viscous liquid was obtained; ¹H NMRindicated that the crude product was a mixture of 1-iodo-4-octylbenzene(69%), 1-iodo-2-octylbenzene (24%), and unreacted 1-phenyloctane (7%).This crude product was used in subsequent preparation of5,11-bis(4-octylphenyl)indolo[3,2-b]carbazole, compound (2), withoutcomplications. ¹H NMR data for 1-iodo-4-octylbenzene (CDCl₃): δ 7.54 (d,J=8.2 Hz, 2H), 6.92 (d, J=8.2 Hz, 2H), 2.53 (t, J=7.7 Hz, 2H), 1.55 (m,2H), 1.20-1.40 (m, 10H), 0.88 (t, J=6.9 Hz, 3H).

A mixture of 1-iodo-4-octylbenzene (16.09 g, 35.1 mmol, 69% of purity)as prepared above, 5,11-dihydridoindolo[3,2-b]carbazole (3.00 g, 11.7mmol), 18-crown-6 (0.62 g, 2.34 mmol), K₂CO₃ (12.94 g, 93.6 mmol),copper (2.97 g, 46.8 mmol), and 1,2-dichlorobenzene (50 mL) was chargedinto an argon-filled 200 mL flask fitted with a condenser. The mixturewas heated under reflux in an argon atmosphere for 24 h. Subsequently,the reaction mixture was cooled down to room temperature, diluted withtetrahydrofuran, and filtered. A viscous liquid, obtained after removalof solvent on a rotary evaporator, was added to 400 mL of methanol withvigorous stirring. The precipitated yellow solid was filtered, washedseveral times with water and methanol, dissolved in 400 mL of hexane byheating, and filtered to remove the insoluble impurities. The filtratewas concentrated to about 50 mL and allowed to cool down to roomtemperature, and then chilled at 0° C. overnight. The crystallizedyellow product was filtered, washed with a small amount of hexane, anddried to yield 5.32 g of 5,11-bis(4-octylphenyl)indolo[3,2-b]carbazole,compound (2), which was subject to train sublimation to obtainelectrically pure compound for OTFT fabrication. DSC showed a meltingpoint at 131° C. on first heating. ¹H NMR (CDCl₃): δ 8.12 (d, J=7.6 Hz,2H), 8.05 (s, 2H), 7.58 (d, J=8.3 Hz, 4H), 7.47 (d, J=8.3 Hz, 4H),7.39-7.40 (m, 4H), 7.19-7.21 (m, 2H), 2.78 (t, J=7.8 Hz, 4H), 1.78(pent, J=7.4 Hz, 4H), 1.30-1.50 (m, 20H), 0.92 (m, 6H); IR (NaCl): 3047,2956, 2923, 2852, 1517, 1451, 1324, 1236, 842, 731 cm⁻¹.

(b) OTFT Device Fabrication and Evaluation

The fabrication and characterization of compound (2) as a semiconductorfor OTFTs were carried out in accordance with the procedures ofExample 1. During vacuum deposition of compound (2), the silicon wafersubstrate was held at room temperature as well as at 50° C.

The vacuum evaporated thin film of compound (2) with the substratetemperature of 50° C. showed predominately crystalline characteristicsas revealed by X-ray diffraction measurement. Sharp diffraction peak inits X-ray diffraction pattern was observed at 2θ=3.62° with the thirdand fifth diffraction peaks at 2θ=10.91° and 18.09°, corresponding to aninterlayer distance of 24.41 Å.

The devices were evaluated in the same manner as in Example 1. Usingtransistors with a dimension of W=5,000 μm and L=90 μm, the followingaverage properties from at least five transistors were obtained:

Devices fabricated with substrates held at room temperature:

Mobility: 0.01-0.02 cm²/Vs

On/off ratio: 10⁵-10⁶.

Devices fabricated with substrates held at 50° C.:

Mobility: 0.07-0.12 cm²/Vs

On/off ratio: 10⁶-10⁷.

Example 3 (a) Synthesis of2,8-dichloro-5,11-didodecylindo[3,2-b]Carbazole (3)

To a well-stirred suspension of 4-chlorophenylhydrazine hydrochloride(24.5 g, 0.137 mol) in ethanol (200 mL) in a 500-mL flask was added asolution of sodium acetate trihydrate (56.34 g, 0.414 mol) in water (100mL), and the resultant mixture was stirred for 15 min at roomtemperature. Subsequently, a solution of 1,4-cyclohexandione (7.67 g,68.4 mmol) in ethanol (50 mL) was added, followed by addition of 50 mLof acetic acid. The reaction mixture was heated at 50° C. for 1 h beforecooling down to 0° C. and maintained there for 1 h. The precipitatedlight yellow crude cyclohexane-1,4-dione bis[(4-chlorophenyl)hydrazone]was filtered, washed with water, air-dried, and added in small portionsto a mixture of acetic acid (75 mL) and sulfuric acid (15 mL, 98%) in a1-L flask with stirring at 10° C. over a period of 10 min, and thenallowed to warm to 25° C. and stirred for 10 min. Subsequently, themixture was heated to about 65° C. until reaction occurred, and furtherstirred at 65° C. for 15 min before cooling down to and stirred at roomtemperature overnight. The product was filtered, washed with methanoland water, and then stirred in 200 mL of boiling methanol for 30minutes, filtered, and dried in vacuo at 50° C. for 5 h to give 4.26 g(22.8%) of 2,8-dichloroindolo[3,2-b]carbazole, which was pure enough forsubsequent preparation of2,8-dichloro-5,11-didodecylindolo[3,2-b]carbazole, compound (3).

¹H NMR (DMSO-d6): 11.34 (s, 2H), 8.33 (d, J=1.9 Hz, 2H), 8.21 (s, 2H),7.46 (d, J=8.6 Hz, 2H), 7.38 (dd, J=8.6 Hz, J₂=1.9 Hz, 2H).

50% aqueous NaOH solution (4 mL) was added to a well-stirred mixture of2,8-dichloroindolo[3,2-b]carbazole (0.65 g, 2 mmol) as prepared above,benzyltriethylammonium chloride (90 mg, 0.4 mmol), and 1-bromododecane(1.99 g, 8 mmol) in DMSO (20 mL) in a 100-mL flask, and the resultantmixture was stirred at room temperature for 1 h and then at 50° C. for 4h. Subsequently, the mixture was poured into MeOH (200 mL), and theprecipitated yellow solid was filtered and washed 3 times each withwater, N,N-dimethylformamide, methanol, and acetone, yielding 1.16 g(88.0%) of 2,8-dichloro-5,11-didodecylindolo[3,2-b]carbazole, compound(3), after drying in vacuo. It was then subject to train sublimation toobtain electrically pure samples for OTFT fabrication.

¹H NMR (CDCl₃): δ 8.15 (d, J=1.9 Hz, 2H), 7.94 (s, 2H), 7.43 (dd, J=8.6Hz, J₂=1.9 Hz, 2H), 7.32 (d, J=8.6 Hz, 2H), 4.38 (t, J=7.2 Hz, 4H), 1.92(pent, J=7.2 Hz, 4H), 1.30-1.50 (m, 36H), 0.87 (t, J=6.8 Hz, 6H). IR(NaCl): 2947, 2920, 2846, 1513, 1464, 1440, 1316, 1127, 1068, 847, 787,726 cm⁻¹.

(b) OTFT Device Fabrication and Evaluation

The fabrication and characterization of compound (3) as a semiconductorfor OTFTs were carried out in accordance with the procedures ofExample 1. During vacuum deposition of compound (3), the silicon wafersubstrate was held at three different temperatures: room temperature,50° C., and 70° C.

The vacuum evaporated thin film of compound (3) with the substratetemperature of 70° C. showed predominately crystalline characteristicsas revealed by X-ray diffraction measurement. Sharp diffraction peak inits X-ray diffraction pattern was observed at 2θ=4.14° with the secondand third diffraction peaks at 2θ=8.28° and 12.42°, corresponding to aninterlayer distance of 21.3 Å.

The devices were evaluated in the same manner as in Example 1. Usingtransistors with a dimension of W=5,000 μm and L=90 μm, the followingaverage properties from at least five transistors were obtained:

Devices fabricated with substrates held at room temperature:

Mobility: 0.02-0.03 cm²/Vs

On/off ratio: 10⁵-10⁶.

Devices fabricated with substrates held at 50° C.:

Mobility: 0.06-0.085 cm²/Vs

On/off ratio: 10⁶-10⁷.

Devices fabricated with substrates held at 70° C.:

Mobility: 0.085-0.14 cm²/Vs

On/off ratio: 10⁷.

Example 4 (a) 5,11-Bis(4-methylphenyl)indolo[3,2-b]carbazole

A mixture of 5,11-dihydridoindolo[3,2-b]carbazole (2.56 g, 10 mmol),18-crown-6 (0.52 g, 2.0 mmol), K₂CO₃ (11.06 g, 80 mmol), 4-iodotoluene(6.54 g, 30 mmol), copper (2.54 g, 40 mmol), and 1,2-dichlorobenzene (40mL) was charged into an argon-filled 200 mL flask fitted with acondenser. The mixture was heated under reflux in an argon atmospherefor 24 h. Subsequently, the reaction mixture was cooled down to roomtemperature, diluted with toluene, and filtered. The solid was stirredin 50 mL of N,N-dimethylformamide and 50 mL of 2N HCl was added dropwiseand the mixture was stirred for 30 min. The yellow suspension wasdecanted and filtered to give a yellow solid. The solid was then stirredin a mixture of DMSO (200 mL) and 20% NaOH (50 mL) for 30 min and thenfiltered. The solid was washed with deionized water, methanol, and,dried to yield 1.65 g of 5,11-bis(4-methylphenyl)indolo[3,2-b]carbazole,which was then subject to train sublimation to obtain electrically purecompound for OTFT fabrication. DSC showed two endotherms at 305° C. and322° C. on first heating. ¹H NMR (CDCl₃): δ 8.11 (d, J=7.8 Hz, 2H), 8.04(s, 2H), 7.57 (d, J=8.3 Hz, 4H), 7.48 (d, J=8.3 Hz, 4H), 7.39 (d, J=3.7Hz, 4H), 7.21 (m, 2H), 2.54 (s, 6H); IR (NaCl): 3034, 2916, 1606, 1515,1451, 1323, 1235, 1191, 819, 741 cm⁻¹.

(b) Device Fabrication and Evaluation

The fabrication and characterization of5,11-bis(4-methylphenyl)indolo[3,2-b]carbazole as a semiconductor forOTFTs were carried out in accordance with the procedures of Example 1.During vacuum deposition, the silicon wafer was held at three differenttemperatures: room temperature, 50° C. and 70° C.

X-ray diffraction measurements were done on the vacuum evaporated thinfilm of 5,11-bis(4-methylphenyl)indolo[3,2-b]carbazole with thesubstrate temperature of 70° C. Only featureless broad diffraction peakswere present in the XRD pattern, indicating its amorphous nature.

The devices were evaluated in the same manner as in Example 1 usingtransistors with a dimension of W=5,000 μm and L=90 μm. All devices with5,11-bis(4-methylphenyl)indolo[3,2-b]carbazole semiconductor exhibitedmobility of about 10⁻⁵ cm²/Vs regardless of the substrate temperature.

Embodiments of the present OTFT are illustrated in Examples 1 through 4.It is noted that the mobilities of the devices in Examples 1, 2, and 3having a predominately crystalline semiconductor layer are higher thanthe mobility of the device in Example 4 having a predominately amorphoussemiconductor layer. Other embodiments of the present OTFT (containing apredominately crystalline semiconductor layer or a predominatelyamorphous semiconductor layer) may provide similar or betterperformance.

1. A substituted indolocarbazole of structure (A):

wherein each R is independently alkylaryl, and wherein theindolocarbazole is peripherally substituted by one or more halogensubstituents.
 2. A substituted indolocarbazole of claim 1, havingstructure (A-1):

wherein each R′ is independently halogen.
 3. The substitutedindolocarbazole of claim 1, wherein each R group is alkylphenyl.
 4. Thesubstituted indolocarbazole of claim 3, wherein the two R groups are thesame and the alkyl has from about 6 to about 18 carbon atoms.
 5. Thesubstituted indolocarbazole of claim 1, having structure (5):